One or more deformations present within a sheet of a medium to be printed can cause serious reliability problems in a printing system, such as an inkjet printing system, where there is only a small gap between a sheet transport mechanism and an image forming device or printing head of the printing system. If the sheet to be printed touches the image forming device or the printing head as a result of such a deformation, this can lead to print quality degradation and/or to a sheet jam in the machine. To achieve high print quality in an inkjet printing system, the distance between the printing heads and sheet to be printed should be kept small. Because of this small distance (print gap) the print heads are easily touched by the sheets as they pass. Accordingly, even small defects like dog ears, wrinkles, tears etc. can cause a so-called “head touch”, which can degrade print quality, cause nozzle failure, or even sheet jams.
To address these issues, systems have been developed which employ a proofing device capable of identifying sheet deformations and rejecting sheets that contain such deformations. U.S. Pat. No. 8,419,144 B2 discloses such a proofing device, wherein multiple emitters are positioned alongside a transport path of the printing system, such that the emitters emit optical beams over the surface of the medium. A deformation in the sheet may then be detected as it passes through one or more of the beam by a corresponding drop in the detected intensity. Drawback of the known proofing device is its incapability to distinguish between different sheet types, especially when these sheets are applied as a continuous stream. The same rejection criterion is applied to all sheets. The accuracy of the defect detection is thereby reduced, as the rejection criteria cannot be attuned to the specific conditions of a sheet, for example to its media type or prior processing of said sheet, without interrupting the printing process.